1. Field of the Invention
The present invention relates to a system and method for distributing digital works, an apparatus and method for reproducing digital works, and computer program products including computer usable mediums in which computer readable programs for the above system, apparatus and methods have been embodied. More particularly, the present invention relates to the control of use of digital works, the control of use of secondary works, and the levy of charges for the use of digital works.
2. Description of the Related Art
In a digital work distribution system, encrypted software and multimedia contents are supplied to users, and the users pay prices conforming to the usage rules presented by sellers, through utilization of credit cards, banks, electronic money, etc. An example of a conventional digital work distribution system is described in Unexamined Japanese Patent Application KOKAI Publication No. H9-138827.
The digital work distribution system described in the above publication includes a distribution center apparatus and a user apparatus. Having made an appropriate usage contract with a user in regard to a work which the user has requested through the user apparatus, the distribution center apparatus encrypts work data representing the work by using a public key of the user, and transmits the encrypted work data to the user apparatus.
The work data sent to the user apparatus has usage conditions affixed thereto. Based on those usage conditions, the use of the work is restricted to a usage such as “reference only”, “editable” or the like. Original data, secondary work data, etc. are encapsulated together to provide a secondary work, which is distributed through the distribution center apparatus as in the case of the original data.
Another work distribution system is also generally well known, in which a work is encrypted and a decipher key of the work is distributed only in the case where a predetermined amount of money has been paid.
Meanwhile, a Kerberos system is also well known, which verifies using a common cipher that authentication information I is information generated by a prover P.
According to this system:    (1) a key distribution center passes, to the prover P, a key “Kc, s” and a ticket in which data, containing the authentication information I and a session key “Kp, v”, has been encrypted with a key of the prover P;    (2) the prover P passes the ticket to a verifier V;    (3) the prover P also passes to the verifier V an authenticator in which the authentication information I has been encrypted with the key “Kc, s”;    (4) the verifier V decrypts the authentication information I and the key “Kp, v” contained in the ticket by using a key Kv, and further decrypts the authentication information I contained in the authenticator by using the key “Kc, s”, and then certifies that the authentication information I contained in the ticket and that contained in the authenticator are identical with each other.
Let it be considered the case wherein the above-described Kerberos system is applied to a digital work distribution system, and the authentication information is replaced by a work, while the session keys are replaced by keys according to usage of the work. In this case, the use of the work can be controlled by distributing the work in an encrypted state and distributing the ticket (not containing the work) at the time of use of the work.
According to another digital work distribution system in which the use of the work is strictly limited, an inhibited usage list that specifies impossible usages is affixed to the work, and the list is interpreted to determine whether a variety of usages can be practiced or not. An example of this kind of digital work distribution system is described in Unexamined Japanese Patent Application KOKAI Publication No. H9-269916.
The digital work distribution system described in the above publication includes an editing apparatus, a distribution server apparatus and an audiovisual apparatus. The editing apparatus edits a work, generates an inhibited usage list, and encapsulates them in a capsular work. The distribution server apparatus distributes the capsular work to the audiovisual apparatus. The audiovisual apparatus opens the capsular work. Based on the inhibited usage list, such hooks as would prevent the interprocess communications of messages are applied in correspondence with a variety of usages to an operating system for the audiovisual apparatus. By so doing, the usages specified in the list are inhibited.
According to another digital work distribution system which enables the copyright of the secondary work to be protected, the author of a secondary work generates a secondary cipher key different from a cipher key used to encrypt the primary work. The secondary work is encrypted with the secondary cipher key and is then distributed. In this case, in order to grasp the authoring of such secondary works, a copyright management center manages their secondary cipher keys. An example of this kind of digital work distribution system is described in Unexamined Japanese Patent Application KOKAI Publication No. H8-272745.
According to the digital work distribution system described in the above publication, a primary work is encrypted with a primary cipher key. In the case of creating and distributing the secondary work which utilizes the primary work, the secondary cipher key is generated and the secondary work is distributed after encrypted with the secondary cipher key. The secondary work is one in which the original data representing the primary work and procedures for creating the secondary work have been encapsulated. The primary cipher key is used to decrypt the encrypted original data representing the primary work, while the secondary cipher key is used to decrypt the entirety of the encrypted secondary work.
There has been proposed a method for making a charge for use of a work on the basis of the monitored frequency of use. An example of a work usage control system employing this method is described in Unexamined Japanese Patent Application KOKAI Publication No. H8-95777.
In the work usage control system described in the above publication, the frequency of use, i.e., the number of times a work has been used, is counted each time a predetermined use of the work is made. The work has the charging information affixed thereto. The charging information has undertaken such a setting as would allow the work to be used a predetermined number of times without charge.
The conventional systems described above have the following drawbacks:
The first drawback is that an author's request and a seller's request as to the control of a distributed work are not satisfactorily reflected. This is because only a fixed usage of capsular work data received by a user is controllable. In the case where the author and the seller wish to control different usages of the work data, they have to make a number of usage conditions and perform encapsulation with respect to each of those usage conditions. However, this cannot be said to be enough to reflect the intentions of the author and seller.
The second drawback is that it is risky to encapsulate the usage conditions and the work together. The reason for this is that although the usage conditions and the work data are stored in a nonvolatile memory, any illicit change can be made to the usage conditions stored in the nonvolatile memory.
The third drawback is that the efficiency of use of the secondary work is low. The reason for this is that according to a method as that described in Unexamined Japanese Patent Application KOKAI Publication No. H8-272725 mentioned previously, the encrypted original work has to be decrypted first and thereafter data added as the secondary work has to be decrypted in the case of using the secondary work. Normally, a work such as an image or a moving picture is excessively large in size. Therefore, if decryption is executed a number of times as in the case of the above method, this will considerably lower the throughput of a reproducing process.
The fourth drawback is that retaining the variety of distributed works is difficult. This is because if a method for making a charge is predetermined according to system or if a charge is made based only on a fixed unit, e.g. the frequency of use, this will entail the difficulty of dealing with a variety of works, such as works (of a pay-per-view type) like movies and works (of such a type as can be used over and over without charge once purchased) like books.